Dicyanamide-intertwined assembly of two new Zn complexes based on N2O4-type pro-ligand: Synthesis, crystal networks, spectroscopic insights, and selective nitroaromatic turn-off fluorescence sensing

Article abstract of DOI:10.1016/j.saa.2021.119612

Two new dicyanamide bridged multinuclear Zn complexes, [Zn₂(L¹)(μ_1,5-dca)₂(μ₁-dca)]_n (1) and [Zn₂(L²)(μ_1,5-dca)₂(μ₁-dca)]_n

Full text of DOI:10.1016/j.saa.2021.119612

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Dicyanamide-intertwined assembly of two new Zn complexes based on
N2O4-type pro-ligand: Synthesis, crystal networks, spectroscopic
insights, and selective nitroaromatic turn-off fluorescence sensing
b
c,
a
Dhrubajyoti Majumdar a,b, Swapan Dey b, , Annu Kumari , Tapan Kumar Pal , Kalipada Bankura ,
⇑
⇑
Dipankar Mishra a,
⇑
a Department of Chemistry, Tamralipta Mahavidyalaya, Tamluk 721636, West Bengal, India
b Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826004, India
c Department of Chemistry, Pandit Deendayal Petroleum University, Gandhinagar 382007, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Two new Zn dicyanamide complexes
were synthesized using N2O4 pro-
ligands.
ꢀ
ꢀ
X-ray diffraction shows complexes of
dicyanamide are 1D templates.
Fluorescence mechanism, detection
Limit (LOD), and KSV were determined
using the Stern-Volmer equation (SV).
Complex 2 detect TNP with high
selectivity and sensitivity in DMF.
The quenching mechanism was
rationalized by (a) electrostatic
interactions, (b) PETvia DFT, and (c)
FRET.
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Two new dicyanamide bridged multinuclear Zn complexes, [Zn2(L1)(m1,5-dca)2(m1-dca)]n (1) and [Zn2(L2)
(m1,5-dca)2(m1-dca)]n (2) have been synthesized using N2O4-based pro-ligands (H2L1 = N,N0-bis(5-bromo-
3-methoxysalicylidenimino)-1,3-diaminopropane, H2L2 = N,N0-bis(3-ethoxysalicylidene)-2,2-dimethyl-1
,3-propanediamine) and characterized by microanalytical and spectroscopic techniques. Both complexes
are stable in solution and solid-state. Thermogravimetric analysis (TGA) findings showed that complexes
are stable at room temperature. Single-crystal X-ray diffraction (SCXRD) has proven that complexes are
identical structures where two zinc metal ions are crystallographically independent. The directional
properties of dicyanamide co-ligands via m1,5 bridging have resulted in different connectivity of zinc
Article history:
Received 8 September 2020
Received in revised form 5 February 2021
Accepted 7 February 2021
Available online 23 February 2021
Keywords:
Schiff base
Zn(II)
Dicyanamide
Fluorescence
Sensing
metal ions leading to 1D templates. SCXRD revealed some notable non-covalent interactions (
p
ꢁ ꢁ ꢁ
p, C-
Hꢁꢁꢁꢁ , and H-bonding) in their solid-state crystal structures. 1–2 have strong fluorescence behaviour over
p
pro-ligands, which may be quenched in the presence of various electron-deficient explosive nitroaro-
matic compounds (epNACs). Complex 2 fluorescence intensity is sharper than 1; hence the former
retained high sensitivity and selectivity for trinitrophenol (TNP). The enhancement of fluorescence mech-
anism, detection limit (LOD), and the quenching constant (KSV) have been calculated using the Stern-
Volmer equation (SV), where the KSV value for TNP is found to be 1.542 ꢂ 104 Mꢃ1. The solution phase
quenching mechanism has been rationalized by (a) electrostatic interactions through charge-transfer
complex, (b) photo-induced electron transfer (PET) by the HOMO-LUMO energy gap via DFT, and (c)
NACs
⇑
Corresponding authors.
E-mail addresses: deyswapan77@yahoo.com (S. Dey), tapan.pal@sot.pdpu.ac.in
(T.K. Pal), dmishra.ic@gmail.com (D. Mishra).
https://doi.org/10.1016/j.saa.2021.119612
1386-1425/Ó 2021 Elsevier B.V. All rights reserved.

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
fluorescence resonance energy transfer (FRET). Finally, complex 2 is applied as a sensor by turn-off fluo-
rescence response to detecting TNP nitroaromatics in the DMF medium.
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
bolstered the quenching mechanism through charge-transfer com-
plex PET and FRET.
In recent decades multinuclear Schiff base complexes with tran-
sition metal ions have been among the most meticulously studied
coordination chemistry topics. In this upbringing compartmental
N2O4-type pro-ligand are very popular because of their handy syn-
thesis method product stability versatility and a wide range of
complexing attraction [1–3]. Zinc metal chemistry is peculiar
because it often occurs in bivalent form solution and metal organ-
ics [4]. Earned to the d10 electronic system, softness borderline nat-
ure (HSAB), second great metal after iron in the mammalian body,
less toxic and carcinogenic over congeners metal cadmium/mer-
cury, Zn coordination chemistry always blooming in literature
[5]. Besides Zn complexes ground states have no crystal field stabi-
2. Experimental section
2.1. Starting materials and instrumentation
All research chemicals and solvents were of handy and reagent
grade used as purchased with no further purification. Zn(OAc)2-
ꢁ2H2O
Sodium
hydroxybenzaldehyde
dicyanamide
(NaN(CN)2)
5-Bromo-3-methoxy-2-hydroxybenzalde
3-ethoxy-2-
hyde 1,3-diaminopropane and 2,2-dimethyl-1,3-propanediamine
were procured from the Sigma Aldrich Company USA. Elemental
analyzes (CHN) were completed on a PerkinElmer 2400 elemental
analyzer. FT-IR and Raman spectra were recorded as KBr pellets
(4000–400 cmꢃ1) using a Perkin–Elmer spectrum RX 1 and BRU-
KER RFS 27 (4000–50 cmꢃ1) model. 1H and 13C NMR spectra were
collected on a Bruker 400 MHz and 75.45 MHz FT-NMR spectrom-
eter operating tetramethyl silane (TMS) as an internal standard in
DMSO d6. We performed Energy-dispersive X-ray spectroscopy
(EDX) experiments on EDX OXFORD XMX N using Tungsten fila-
ment. JEOL Model JSM6390LV analyzed different Scanning Electron
Microscopy (SEM) images. TGA experiments were carried out on a
TGA-5OH analyzer from ambient temperature to 1000 °C at a tem-
perature rating of 30 °C/min in a flowing 10 mL/min under a nitro-
gen atmosphere environment using a platinum cell. We carried
Powder X-ray diffraction (PXRD) measurements by BRUKER AXS
n
lization energy (CFSE) value which is primitive for other d elec-
tronic systems (where n = number of d orbital electrons) [6]. The
variation of coordination number and stereochemical arrangement
is restricted in zinc metal ions but the adaptable geometry is over-
whelmed by the ligand’s steric requirement [7]. Meanwhile the Zn
complex d-d transition cannot take place because of the filled d10
electronic system. Thus the spectroscopy and photochemistry are
for M ? LCT or L ? MCT and ligand–based/inter-configurational
metal-centred transitions [8–11]. Remarkably zinc metal ions bio-
logical role has been explored in many essential hydrolytic
enzymes such as carbonic anhydrase carboxypeptidase A and
phosphatase [12]. The bulk of the Zn complexes exhibit photolumi-
nescence and electroluminescence properties [13]. Furthermore
nitroaromatic compounds (NACs) such as nitrophenol (NP), trini-
trophenol (TNP), 2,4-dinitrotoluene (2,4-DNT), 4-nitrobenzoic acid
(4-NBA), 4-nitrotoluene (4-NT), nitrobenzene (NBZ), and meta-
dinitrobenzene (m-DNB) are explosive and poisonous to the soil
river and groundwater pollutants [14]. Also, 4-NP is a harmful pol-
lutant causing significant human health problems [15]. Therefore,
the NACs are of particular concern because of Homeland’s protec-
tion environment and public safety [16,17]. While the various
research institutes provide numerous explosive detection methods
they are costly and difficult to manage [18,19]. In the prevailing
research scenario, Fluorescence-based NAC detection is one of
the most promising and demanding techniques for explosive
detection for its cost-effectiveness high selectivity portability and
combined application in solution and solid stages [20–22]. Inter-
estingly all major science branches have been actively involved
in designing and implementing coordination polymers (CPs) as
chemical sensors to detect NACs [23]. Flexible dicyanamide is
actively considered in current research work as it shows versatile
coordination motives to enhance CPs formation (Scheme S1) [24–
29]. Numerous CPs have been employed to apply sensors for their
stability, mobile synthetic technique, and structural architecture
flexibility [30–36]. Because of their wide range of applications like
photoluminescence [37], biomedical [38], antibacterial, antibiofilm
[38], and excellent magnetic properties [38], in recent years our
research group has been exploring the chemistry of Zn dicyana-
mide CPs.
GERMANY X-ray diffractometer D8 FOCUS model using Cu Ka-1
radiation. UV–Visible spectra in CH3OH and DMSO solvents (200–
1100 nm) were determined using the Hitachi model U-3501
spectrophotometer.
2.2. Fluorescence quantum yield (U)
Fluorescence quantum yield (
complexes have been resolved using condition (1) where quinine
sulfate an auxiliary norm ( = 0.57 in water) [39]. Equation S1 rec-
U) of ligands and Zn dicyanamide
U
ommended for the lifetime decay calculation of the complexes.
US AS ðAbsÞR n2S
¼
ꢂ
ꢂ
ð1Þ
nR2
UR AR ðAbsÞS
According to Eq. (1) A terms denote the fluorescence area under
the curve; Abs denotes absorbance; n is the medium refractive
index;
U is the fluorescence quantum yield; and subscripts S and
R denote parameters for the studied sample and reference.
2.3. Fluorescence sensing measurement
The stock solution was prepared by dissolving complex 1 and 2
in DMF. The sensing properties of complexes in different solvents
were investigated by adding equal amounts of dicyanamide com-
plexes (10ꢃ5 M) to 3 mL of other solvents including NACs. Experi-
ments of fluorescence titration were carried out by gradual
addition of 20 mL 10ꢃ3 M analyte in DMF to 3 mL 10ꢃ5 M of com-
plex in DMF.
In this endeavour two dicyanamide modulated Zn complexes
[Zn2(L1) (m1,5-dca)2(m1-dca)]n (1) and [Zn2(L2)(m1,5-dca)2(m1-dca)]n
(2) have been prepared and structurally characterized. Solution
phase turn-off fluorescence sensing with various epNACs sensing
mechanisms detection limits and binding constant (KSV) using
the SV equation have been reported. Electrostatic interactions have
2

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
2.4. X-ray crystallographic data collection and refinement
cooling the solution which was collected and air-dried. N,N0-bis(5-
bromo-3-methoxysalicylidenimino)-1,3-diaminopropane (H2L1):
Yield: 0.215 g (85.8%) Anal. Calc. for C19H20 Br2N2O4: C 45.62; H
Light yellow crystals of the Zn dicyanamide complexes were
grown after slow evaporation of methanol solvent in the presence
of a few drops of acetonitrile (ACN). Data on a Bruker SMART CCD
4.03; N 5.60 Found: C 45.66; H 4.08; N 5.62%. IR (KBr cmꢃ1
)
selected bands:
m(C@N) 1648 vs, m(CAOphenolic) 1229 s, m(OAH)
[40] diffractometer using Mo K radiation at k = 0.71073 Å (277–
3430 s, 1H NMR (DMSO d6 400 MHz): d (ppm): 3.75 (s, 3H1),
a
296 K) were collected by selecting four excellent quality crystals.
Here it is necessary to have an observation that judicious crystal
data collection purpose we have applied for some basic programs:
SMART for collecting frames of data indexing reflections and deter-
mining lattice parameters SAINT [41] SADAB [42] and SHELXTL for
space group structure determination and least-squares refine-
ments on F2. The crystal structure was solved by full-matrix
least-squares methods against F2 using the program SHELXL-2014
[43] and Olex-2 software [44]. Non-H atoms (all) were refined with
anisotropic displacement parameters, and all hydrogen positions
were fixed at calculated positions which are refined iso-
tropically. We designed various crystallographic figures using Dia-
mond software. The crystallographic data and full structure refine-
ment parameters are submitted in Table 1.
7.06–7.17 (m, 1H2 1H3), 8.48 (m, 1H5), 13.91 (m,1H4), 3.04–3.75
(s, 2H6), 2.48 (s, 6H7), 13C NMR (DMSO d6 75.45 MHz):
d
(ppm):54.94–56.46 (O-1CH3),117.23–150.18 (Arom-3C-5C),153.71
(7C-OH), 165.77 (8CH@N), 31.40 (ACA(9CH2)2), UV–Vis (kmax CH3-
OH): 229, 292 and 425 nm.
N,N0-bis(3-ethoxysalicylidene)-2,2-dimethyl-1,3-propanedia
mine (H2L2): Yield: 0.178 g (89.4%) Anal. Calc. for C23H30N2O4: C
69.32; H 7.59; N 7.03 Found: C 69.60; H 7.62; N 7.01%. IR (KBr
cmꢃ1) selected bands:
m(C@N) 1638 vs, m(CAOphenolic)1245 s, m
(OAH) 3436 s, 1H NMR (DMSO d6 400 MHz): d (ppm): 1.31–1.34
(s, 3H2), 4.04 (H1), 6.77–7.03 (m, 1H3 1H4 1H5), 8.53 (m, 1H7),
13.90 (m,1H6) 3.99 (s, 2H8) 0.98(s, 6H9),13C NMR (DMSO d6
75.45 MHz): d (ppm):15.2 (2CH3) 64.2–67.0 (O-1CH2),116.3–147.6
(Arom-4C-6C),152.2 (8C-OH), 167.3 (9CH@N), 24.0 (11C-(CH3)236.1
((CH3)2-C-(10CH2)2), UV–Vis (kmax CH3OH): 226, 263 and 420 nm.
2.5. Synthesis of N,N0-bis(5-bromo-3-methoxysalicylidenimino)-1,3-
diaminopropane and N,N0-bis(3-ethoxysalicylidene)-2,2-dimethyl-
1,3-propanediamine
2.6. Synthesis of complex [Zn2(L1)(m1,5-dca)2(m1-dca)]n (1)
Zinc acetate dihydrate (0.219 g, 1 mmol) was dissolved in 25 mL
of methanol. Then a methanolic solution of the ligand (H2L1)
(0.502 g, 1 mmol) was added to it followed by the drop-wise addi-
tion of an aqueous solution of Sodium dicyanamide ((0.0891 g,
1 mmol). The resultant mixture could stir for 1 h at 45 °C. Then 5
drops of acetonitrile (ACN) were added and it continued stirring
for the next 1 hr. The bright yellow solution was filtered and kept
refrigerated for crystallization. After a few days block size yellow-
coloured single crystals suitable for SCXRD were obtained. Crystals
were isolated by filtration and air-dried. Yield: 0.457 g (59.8%)
Anal. Calc. for C23H18 Br2N8O4Zn2: C 36.30; H 2.38; N 14.72. Found:
Pro-ligands (H2L1-H2L2) have been synthesized in our labora-
tory following the literature method (Scheme 1) [45]. The reflux
condensation of 5-bromo-3-methoxy-2-hydroxybenzaldehyde
(0.231 g, 1 mmol) with 1,3-diaminopropane (0.0371 g, 0.5 mmol)
in (50 mL) methanol at 80 °C for 3 h prepared H2L1. A similar pro-
cedure was used for the synthesis of H2L2 except 2,2-dimethyl-1,3-
propanediamine (0.0511 g, 0.5 mmol) condensed with 3-ethoxy-2-
hydroxybenzaldehyde (0.166 g, 1 mmol). The solvent was removed
under vacuum and the yellow powder product separated out upon
C 36.26; H 2.40; N 14.75%. IR (KBr cmꢃ1) selected bands:
1580 s, (C„N) 2281 m, 2170 s,
(ArAO),1243 FT-Raman (cmꢃ1
selected bands: (C@N) 1632 s,
(C„N) 2286 s, 2218 m, 1H NMR
m(C@N)
m
m
)
Table 1
m
m
Crystal data and structure refinement parameters.
(DMSO d6 400 MHz): d (ppm): 3.48 (s 3H1), 7.17–7.31 (m, 1H2
1H3), 9.85 (m, 1H4), 3.48–3.84 (s, 2H5), 1.94–2.02 (s, 6H6), UV–
Vis kmax (DMSO): 374 nm.
Formula
C23H18 Br2N8O4Zn2 (1) C27H28N8O4Zn2 (2)
M/g
761.01
659.31
Crystal system
Space group
Monoclinic
C 1 2/c 1
17.442(2)
22.398(2)
14.2406(17)
97.160(6)
5519.9(12)
8
Monoclinic
P 1 21/c 1
8.7023(3)
18.8101(7)
17.6730(7)
92.107(2)
2892.91(19)
4
2.7. Synthesis of complex [Zn2(L2)(m1,5-dca)2(m1-dca)]n (2)
a/Å
b/Å
c/Å
b (°)
V/Å3
Z
We obtained 2 in a similar procedure as in the case of 1 using
ligand (H2L2) (0.398 g, 1 mmol) in methanol. Block single size crys-
tals suitable for SCXRD were got after a few days. Filtration and iso-
lated crystals air-dried. Yield: 0.778 g (59.3%) Anal. Calc. for
q
l
c/gcmꢃ3
1.831
4.679
1.514
1.706
/mmꢃ1
C
27H28N8O4Zn2: C 53.15; H 4.31; N 12.82. Found: C 53.11; H
4.35; N 12.90%. IR (KBr cmꢃ1) selected bands:
(C@N) 1621 vs,
(C„N) 2208 s, 2178 m,
(ArAO),1225 FT-Raman (cmꢃ1) selected
bands: (C@N) 1630 vs, m
(C„N) 2278 m, 2239 vs, 1H NMR
F (000)
2992
1352
Cryst size (mm3)
h range (deg)
Limiting indices
0.065 ꢂ 0.045 ꢂ 0.033 0.078 ꢂ 0.061 ꢂ 0.045
m
m
m
2.171 to 25.500
ꢃ21 ꢄ h ꢄ 21
ꢃ27 ꢄ k ꢄ 27
ꢃ17 ꢄ l ꢄ 17
81,448
2.45 to 22.65
ꢃ10 ꢄ h ꢄ 9
ꢃ20 ꢄ k ꢄ 22
ꢃ21 ꢄ l ꢄ 21
20,636
5381 [Rint = 0.0682
Rsigma = 0.0741]
1.000
m
(DMSO d6 400 MHz): d (ppm): 1.47 (s, 3H1), 4.17 (H2), 6.30–7.16
(m, 1H3 1H4 1H5), 8.44 (m, 1H6), 3.35–3.76 (s, 2H8), 0.95 (s, 6H9),
UV–Vis kmax (DMSO): 358 nm.
Reflns collected
Ind reflns
5135 [Rint = 0.0981
Rsigma = 0.0350]
0.999
Completeness to h (%)
3. Results and discussion
Refinement method
Full-matrix-block
least-squares on F2
5135/0/354
1.076
R1 = 0.0480
Full-matrix-block
least-squares on F2
5381/6/374
1.074
R1 = 0.0555
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices
3.1. Synthetic rationalization
N2O4-type pro-ligands were synthesized according to the
method stated (Scheme 1) [45]. Zn complexes derived from pro-
ligands were prepared in moderate good yield by taking the fol-
lowing more familiar in situ procedure where (1:1:1 M) of zinc
acetate dihydrate, respective Schiff base ligands, and Sodium
[I > 2h(I)]
R indices (all data)
wR2 = 0.1271
R1 = 0.0667
wR2 = 0.1382
1.565 and ꢃ0.754
wR2 = 0.1316
R1 = 0.1036
wR2 = 0.1601
0.983 and ꢃ0.488
Largest diff. peak and hole
(eꢁÅꢃ3
)
3

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
Scheme 1. Synthetic Scheme for pro-ligands and formation of complexes 1–2.
dicyanamide in minimum volume aqueous methanol solution
under stirred and refluxed condition (Scheme 1). Keeping in mind
that single crystals were not grown in methanol. Few drops of ace-
tonitrile were used to develop better quality crystals suitable for
SCXRD (Scheme 1). Two complexes are iso-structural 1D polymer
confirmed by SCXRD being formulated as [Zn2(L1)(m1,5-dca)2(m1-
dca)]n (1) and [Zn2(L2)(m1,5-dca)2(m1-dca)]n (2). Ligands and com-
plexes are characterized by elemental analysis and different spec-
troscopic techniques. The compartmental N2O4 pro-ligands
disclose basic imitators since it comprises two imines two phenols
and two alkoxy groups. It produces an N2O2 imine chelating posi-
tion after deprotonation which is trapped by Zn ions in the pres-
ence of dicyanamide co-ligands (Scheme S2) [46].
shift of the absorption bands. The spectral domain also supports
the coordination mode of the pro-ligands with zinc metal ions.
The credit of d10 configuration and diamagnetic nature of zinc
metal ion no metal centric d-d broad absorption band has been
allocated. Further the UV–Vis spectra of ligands and complexes
have been checked in the different polarity of solvents (Figs. S4
and S5). The solvent exerts an effect on the electronic absorption
alerting their forms and maxima spectral positions. We turned
our attention to study photophysical properties as all the synthe-
sized compounds were prepared in a pure way and in the crys-
talline state. To that end we picked solvents like MeOH, HCl in
MeOH, TEA (triethylamine) and DMSO. Ligands absorptions spectra
were assigned with the maxima appearing between 410, and
420 nm depending on the solvent’s polarity. Thus an improvement
in media polarity led to a redshift of the overall absorption maxima
when solvent shifting from MeOH to TEA showing that the ligands
ground state was considerably polar. Also solvent like MeOH spec-
trum remains unchanged but minor changes were observed in HCl
in MeOH. Unlike ligands UV–Vis spectra of 1 and 2 are almost
remained unchanged in the presence of TEA but significantly
affected by HCl in MeOH which is hypsochomically shifted to
356–326 nm [51]. The only remarkable changes were observed
in the non-polar DMSO solvent. The NMR spectroscopic tool also
characterized the ligands and the Zn complexes. The experimental
section presented 1H and 13C NMR spectral data of the synthesized
compounds. Free ligands in the region d 5.0–8.0 ppm no broad
peak were found suggesting the free ANH2 group absence. The
phenolic (OH4/OH6) protons correspond to the defined broad peak
at d 13.91 and 13.90 ppm. The protons (H5/H7) attached to the
imino carbon are downfield shifted (d 8.48–8.5 ppm) [52] because
of the influence of the combined effect of phenolic AOH and imino
N groups in its close vicinity. The peaks near the range d 7.06–7.17
and 6.77–7.03 ppm correspond to the aromatic protons (H2-H3/H3-
H5). The three methyl protons (OCH31) in (H2L1) attached to the aro-
matic oxygen appear at d 3.75 ppm whereas in (H2L2) (CH32) and (O-
CH12) appear at 1.31–1.34 and 4.04 ppm. Protons (H9) in (C-(CH3)2
group appear at d 0.98 ppm. Since methylene protons (H6/H8) being
very close to the imino nitrogen they are de-shielded and appear at d
3.04–3.75 ppm and 3.99 ppm (Fig. S6). Further 13C NMR spectra of
ligands showed the typical azomethine (CH@N) carbons at 167.77–
176.30 ppm (Fig. S8). The other 13C important NMR peaks for
3.2. Spectral characterization
The IR spectroscopy characterized pro-ligands and zinc dicya-
namide complexes. The characteristic imine (C@N) stretching
vibration of synthesized pro-ligands was located within the range
1648–1638 cmꢃ1 [47] (Fig. S1). The stretching vibration bands FT-
IR and Raman (C@N) in 1–2 are moved to 1580–1621 cmꢃ1 (for IR)
(Fig. S2) and 1632–1630 cmꢃ1 (for Raman) (Fig. S3). These spectral
data support the mode of coordination of the imine nitrogen atom
to the zinc metal centre [48]. 1–2 displayed strong bands at 2170,
2281 cmꢃ1 (1) and 2208, 2178 (2) (for FT-IR) and 2286, 2218 cmꢃ1
(1) 2278, 2239 cmꢃ1 (2) (for FT-Raman) [37,38]. These spectral val-
ues are due to the
m(dca) bridging mode. ArAO stretching fre-
quency near 1243–1225 cmꢃ1 is identical with previously
reported Salen-type ligands [49]. The IR and FT-Raman spectra
were extensively compared with reported Zn complexes concern-
ing the bridging mode of dicyanamide co-ligands (Tables S1 and
S2). The UV–Vis absorption spectra of the ligands and complexes
have been performed in methanol and DMSO solvent. Ligands
exhibit three distinct bands at 229, 292, 425 nm and 226, 263,
420 nm (Fig. S4) respectively which are assigned for
* type transitions whereas 1–2 showed a strong single ligand–
based UV domain at 374 and 358 nm (Fig. S5) because of the
p ? p*/n ?
p
L ? M charge-transfer transition (
p ? p*/n ? p*) [48,50]. This
spectral feature is identical to Schiff base N,N0-bis(salicylidene)-1,
3-diaminopentane [51].The presence of electron-withdrawing bro-
mine and electron-donating ethoxy group result in a bathochromic
4

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
(H2L1) observed at 54.94–56.46 (O-CH3), 117.23–150.18 (Arom-C),
153.71 (CAOH), 165.77 (CH@N), 31.40 (AC-(9CH2)2), and for (H2L2)
15.2 (2CH3), 64.2–67.0 (O-1CH2),116.3–147.6 (Arom-4C-6C),152.2
(8C-OH), 167.3 (9CH@N), 24.0 (11C-(CH3)2 ,36.1 ((CH3)2-C-(10CH2)2)
ppm. The coordination mode of azomethine nitrogen was assigned
to the downfield shift of the azomethine proton signal from 8.48
to 8.53 (ligands) to 9.85–8.44 ppm for 1–2. The OH proton (OH4/
OH6) signal at the ligands disappeared in the 1H NMR spectra of
the complexes showing deprotonation and coordination of the oxy-
gen with zinc metal ion [53] (Fig. S7).
(ORTEP) crystallographic asymmetric unit contains two crystallo-
graphically independent zinc metal ions (full occupancy) one
deprotonated pro-ligand [L1]2ꢃ and two dicyanamide co-ligands
(Fig. 1) with
a 1D polymeric structure having the formula
[Zn2(L1)(m1,5-dca)2(m1-dca)]n (1). The asymmetric unit is electroni-
cally neutral. The charge of the two zinc metal ions is balanced
by the contribution of two anionic charges from ligand [L1]2ꢃ and
two anionic charges from dicyanamide. The selected bond dis-
tances (Å) and angles (°) are provided in the supporting informa-
tion (Table S4).
The pro-ligand configuration envisages that it contains two
tetradentate pockets which are comprised of O2N2 (pocket1) and
O4 (pocket2) (Fig. 2a). The two OH groups present in the ligand
are deprotonated during reaction conditions and subsequently
retain the two zinc metal ions firmly through g2 fashion. Thus
the coordination number of each zinc metal ions are provided by
the pro-ligand are four satisfied from the pocket-forming atoms
and forms a dimeric zinc metal cluster [Zn2(O)4(N)2] (Fig. 2b).
Apart from that the coordination activity furnished by the
ligand dicyanamide co-ligand further attacked each zinc metal
ion of the dimeric metal cluster almost perpendicular fashion just
like an insect trunk (vide supra). Further examination of the crystal
structure of 1 discloses the co-ligand dicyanamide helps to grow
the asymmetric unit one-dimensionally (Fig. 3a) which is then
3.3. SEM-EDX and PXRD
The chemical composition of complex 1–2 was analyzed using
EDX. We performed EDX for 1–2 although two complexes are 1D
templates. The EDX profile confirmed the chemical composition
where the weight percentage (%) contribution of the elements sub-
mitted in Table S3. The calculated and EDX values of essential ele-
ments present in 1–2 are nearly good in agreement. EDX profile
contained only predictable elements and other elements are not
impurities. The empirical formula of 1–2 formulated as C23H18Br2-
N8O4Zn2 and C27H28N8O4Zn2. EDX profile represents the highest
peak of C and subsequently O, Zn which the empirical formula fur-
ther agreed upon. Further SEM supports important structural mor-
phological features. SEM images characterized the size and
morphological structure. The micrograph of SEM explores the mor-
phology of 1 which is plate shape (Fig. S9). Further the Zn com-
plexes PXRD patterns were recorded at room temperature within
the range (2h = 4–50°) at a wavelength (k) of 1.54 Å. Because of
their crystalline existence Fig. S10 displayed well defined sharp
PXRD peaks. The experimental PXRD patterns of the complexes
bulk materials are like the patterns simulated from SCXRD data
(CIF) obtained from Cambridge crystallographic data centre (CCDC)
Mercury software consisting that single crystals and bulk material
are the same. The PXRD analytical results will confirm the phase
purity of each bulk sample.
undergoing several numbers of non-covalent interaction (
CAHꢁꢁꢁꢁ and H-bonding) (Table 2) (Fig. 3b) finally offers to the for-
mation of the two-dimensional polymeric framework.
pꢁ ꢁ ꢁp
p
The pro-ligand symmetrically linked to the zinc metal ions via
ƞ2-oxygen and ƞ1-nitrogen atoms to construct a bimetallic metal
cluster (Fig. S12) where one of the dicyanamide acts as a bridge
via
l
2-ƞ1:ƞ1 fashion (Fig. S12) between two bimetallic clusters
leading to the formation of 1D polymer (Fig. 3) and other co-
ligand dicyanamide simply attached to the metallic clustered. This
type of dicyanamide bridging (
ously reported Zn dicyanamide complexes [38,54,55]. Further the
Addison structural parameter ( ) [55] value helps to determine
the correct stereochemical environment around the metal ions
(Zn1 and Zn2) ( are the two largest
= (b ꢃ
)/600 where b and
angles around the central atom: = 0 for perfect square pyramidal
and 1 for a perfect trigonal geometry). The calculated tau ( ) value
l1,5) fashion is identical with previ-
s
s
a
a
3.4. Thermogravimetric analysis
s
s
Under a nitrogen atmosphere the TGA of both the compounds
was performed by applying TGA techniques. The thermal beha-
viour of 1 showed the continuing disintegration of the complex
with an increase in the temperature. In the temperature range
200–585 °C the weight loss of about 8.1% (calculated 8.6%) which
is for the removal of one dicyanamide co-ligand and on a further
increase in the temperature the second dicyanamide removed
along with decomposition of 1 (weight loss 17.2%) (Fig. S11A).
The thermal behaviour of 2 is a little different from 1. At the tem-
perature of 100 °C the TGA showed 4.7% weight loss (calculated 5%)
might be because of the removal of one dicyanamide co-ligand. On
further increase in the temperature no sharp plateau was obtained
and continuous disintegration of the complex which shows at high
temperature (~370 °C) upon removal of another dicyanamide co-
ligand weight loss 10.5% (calculated10%) the decomposition of 2
takes place (Fig. S11B).
of 1 compared with the literature reported Zn dicyanamide com-
plexes (Table S5). Each zinc metal ions are five coordinated square
bipyramidal (
and 2O (Fig. 4).
Interestingly complex 2 crystallizes in the monoclinic space
group P21/c (Z = 4) with asymmetric unit formula [Zn2(L2)(m1,5
s = 0.01) which is fulfilled by binding action from 3N
-
dca)2(m1-dca)]n (2) (Fig. 5). Like in 1 complex 2 is also containing
bimetallic zinc clustered. Further crystallographic investigation
4. Single-crystal X-ray structure
4.1. Crystal structure of 1 and 2
When pro-ligand (H2L1) reacted with zinc metal ion in the pres-
ence of dicyanamide co-ligand in a mixture of MeOH + ACN solvent
1 has resulted. The exposure to SCXRD of the block size light yellow
crystal discloses that the compound crystallizes in the monoclinic
space group (C2/c, Z = 8). The Oak Ridge Thermal Ellipsoid Plot
Fig. 1. The ORTEP diagram of 1 (30% ellipsoid probability of the independent
crystallographic unit).
5

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
that ligands are week fluorescence activity over complexes.
Fig. 6 displayed ligands upon photoexcitation at 330 nm exhibits
maximum fluorescent emission at 421 and 413 nm. 1–2 under
identical photoexcitation shows maximum fluorescence emission
at 456 and 482 nm with the prominent emission peak. Improving
fluorescence emission intensity of 1–2 over ligands is likely due
to N/O-donor centres effective coordination with the zinc metal
ion increasing the conformational rigidity via chelation effect
CHEF (Chelation enhanced fluorescence) followed by loss of
energy by radiation. The active fluorescence of 1–2 relative to
the ligands may be difficult or impossible to oxidize or reduce
due to the d10 configuration zinc metal ion. This fluorescence nat-
Fig. 2. (a) The perspective view of the two different tetradentate pockets in (H2L1)
and (b) dimeric zinc metal clustered in 1.
ure can be assigned to the intra-ligand (
p ? p*) transition or
L ? M charge-transfer referred to as CHEF [57–62]. 2 emission
wavelengths stronger (red shifted) compared to the 1 because
of the ligand [H2L2] N/O-donor effective coordination with zinc
divulges that 2 is initially formed a 1D structure that then under-
goes numerous non-covalent interactions (Table 3) and finally
afforded a two-dimensional polymeric network (Fig. S13). The
metal ions Zn2 and Zn1 are four coordinated tetrahedral and five
coordinated square bipyramidal (ґ=0.09) geometry and the respec-
tive coordination numbers are satisfied from the binding of 2O, 2N
and 3N, 2O (Fig. S14) respectively. The ligand (H2L2) displayed the
metal ion which reduces energy gap between
p and p* of the
ligand thereby making 2 more stables and increases emission effi-
ciency. The ligands quantum yield (ɸ) is measured in DMF and
solid-state but it is shallow under solid-state environments. The
low quantum yield of ligands is related to the conjugated pheno-
lic moiety in the reference ligands due to the fast PET from the
nitrogen lone pair [63,64]. Some bridging Zn complexes with
dicyanamide displayed identical fluorescence behaviour
[37,38,54,56]. Remarkably ligands display chrematistic fluores-
cence emission; thus solid-state fluorescence spectroscopic stud-
ies were conducted for 1–2. Under solid-state photoexcitation at
a similar wavelength shows photoluminescence with the promi-
nent emission intense peak at 419 and 438 nm (Fig. S15). This
binding fashion
mide exhibited two types of binding modes like
l
2- ƞ0: ƞ2: ƞ1: ƞ1: ƞ2: ƞ0 and the co-ligand dicyana-
1- ƞ1 and 2- ƞ1:
l
l
ƞ2 respectively (Fig. S12). Although both complexes are iso-
structural 1D templates yet their coordination number of the zinc
metal ions are different. Therefore the comparison followed
between two complexes regarding the coordination number
attainment of zinc metal ions. In each complex the Zn1 metal ion
present in pocket 1 is five coordinates (Fig. S14]. The Zn2 metal
ion is present in pocket 2 and shows five coordination in 1 and four
coordination in 2 (Fig. 4 and Fig. S14). The less coordination of Zn2
in 2 is probably due to the more flexible and bulky ethyl groups
which create steric crowding and prevents the oxygen atom of
the –OEt group from binding with the Zn2 [51,54]. The binding
mode of dicyanamide is comparable with binuclear Salen com-
plexes [54]. Herein the distance between the two zinc metal ions
is 3.148 Å and 3.143 Å in 2 are relative to other Zn—Zn separation
of double phenoxo-bridged Zn2 complexes [37,38,54,56]. All the
bond angles and distances are like those reported in the literature
(Table S6).
electronic transition is for intra-ligand (
p ? p*) type transition.
Therefore fluorescence behaviours of the Zn complexes are more
active in the solvent than in the solid-state environment. The
maxima fluorescence emission of 2 under solid-state is signifi-
cantly red shifted than 1. Therefore we conclude that fluorescence
behaviours are ligand centred in DMF while in solid-state it is
crystal-dependent [65]. Also the time-resolved fluorescence mea-
surements have been undertaken in DMF (Fig. S16A and B) and
solid-state (Fig. S17 and B) for a better understanding of the emit-
ting properties of the complexes. The fluorescence lifetime has
been measured upon excitation at 370 nm. The decay profile will
be best fitted to tri-exponential nature (with acceptable compara-
ble v2 values close to 1) like other reported Zn complexes. No
expected lifetimes trend was observed in DMF and solid-state
where complexes exhibit unusual decay times having maximum
(95%) and minimum (0.6%) contribution in amplitude (Table 3).
The measured lifetime values further examined the maximum
significance in solid-state of excited states stabilities compared
to DMF.
5. Photoluminescence
5.1. Steady-state and time-resolved fluorescence
Photoluminescence properties of the ligands and 1–2 moni-
tored in DMF solvent (Table 4). The emission spectra revealed
Fig. 3. (a) Two one dimensional layer parallel run in 1 and (b) non-covalent interaction between the two 1D layer.
6

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
Table 2
emission peak at 456 nm upon excitation at 370 nm and 2 emits
at 482 nm when excited at 370 nm. Though both compounds are
fluorescent the fluorescence intensity of 2 is much larger than 1
almost 6-fold under similar experimental conditions because of
the heavy bromine atom effect which increases the chance of inter-
system crossing in 1 results in lower fluorescence intensity [68].
Also because of the same reason the pro-ligand (H2L1) (having
heavy bromine atom) is almost non-fluorescent whereas the ligand
(H2L2) is fluorescent (Fig. 6a). However an increase in the intensity
of 2 compared to the pro-ligands due to strong synergistic interac-
tion among metal and organic ligands [68]. With nitrobenzene
(NBZ) as a solvent both the complex’s fluorescence intensity
quenches (Fig. 7a and Fig. 7b). This experiment showed that both
the complexes are sensitive to the detection of NBZ. Inspired by
such an important conclusion next we verified the detection of
other NBZ congeners and other nitroaromatics compounds consid-
ered to be explosive. Since 2 is strong fluorescent than 1 (vide
supra) we carried out all the sensing experiments with 2. The dif-
ferent nitroaromatics compounds were used are nitrophenol (NP),
trinitrophenol (TNP), 2,4-dinitrotoluene (2,4-DNT), 4-nitrobenzoic
acid (4-NBA), 4-nitrotoluene (4-NT), and m-dinitrobenzene (m-
DNB), and the experimental results showed the different amount
of quenching takes place. Maximum quenching was observed in
the case of TNP upon addition of the same amount of nitro analytes
Possible non-covalent interaction parameters.
Components
Bond distance (Å)
Bond angle (°)
C19-H19AꢁꢁꢁꢁꢁN5
C16-H16ꢁꢁꢁꢁꢁBr
3.208(1)
3.883(5)
3.719(4)
3.174(8)
3.837(4)
3.883(8)
3.833(4)
3.196(5)
149.970(5)
80.740(4)
74.831(5)
125.537(4)
109.161(2)
80.740(4)
91.429(2)
99.952(4)
C14-H14ꢁꢁꢁꢁ
p
C19-H19CꢁꢁꢁꢁꢁBr
ꢁꢁꢁꢁꢁ
C16-H16ꢁꢁꢁꢁꢁ
ꢁꢁꢁꢁꢁ
C19-H19AꢁꢁꢁꢁN8
p
p
p
p
p
p
5.2. Solvatochromism
We studied ‘solvatochromism’ after dissolving 1 and 2 in differ-
ent solvents (Fig. 7a and Fig. 7b) in which two significant proper-
ties of solvents are dielectric constant and H-bonding potential.
Hence solvent variation greatly affected ground and excited from
the complexes so that size of the energy gap between them
changes as polarity solvent changes. Therefore the band’s intensity
and shape will be altered on passing from polar to less polar sol-
vent emission spectrum position. Additionally positive and nega-
tive ‘solvatochromism’ reflected red (if the solvent polarity
increased) and a blue shift in the fluorescence spectrum. Thus in
our present study the emission spectra of 1–2 solvents like
nitrobenzene (NBZ), CHCl3 ,DMF, DMSO, EtOAc, CH3CN, and MeOH
were taken in different polarity. Solvent effects exposed large dif-
ferences between polar and non-polar solvents on the electronic
fluorescence emission spectra [66]. Unlike 1 in the presence of
polar solvents CH3CN and MeOH, 2 emission maxima at 463–
468 nm whereas non-polar solvents such as DMF and DMSO have
a more redshifted emission band to 482–487 nm. Such variations
are of minor importance for other solvents. Therefore with increas-
ing solvent polarity 2 fluorescence emission bands are shifted less
red [67]. This finding indicates a decrease of an excited state dipole
moment than that of the ground state [67]. The above discussion
reveals the transition from polar solvent to less polar fluorescence
300 lL (Fig. 8a and Fig. 8b), and Fig. S18–S22).
Therefore the quenching constant after the addition of different
analytes was calculated using the SV equation (I0 ꢃ I)/I0 ꢂ 100%
where I0 and I are the initial intensity and intensity after the addi-
tion of analytes of the complex. It was manifested the quenching
efficiency by TNP is 93.99% and the quenching efficacy of the nitro
analytes are in the following order: TNPꢅ4-NBA > NP > m-DNB >
NBZ > 2,4-DNT. Considering such high sensitivity of 2 towards
TNP the titration experiment has been conducted upon the gradual
addition of TNP and simultaneously measured the fluorescence
intensity. The quenching efficacy upon addition of a different
amount of TNP is 39.13% for 60
200 L and 93.9% for 300 L. These quenching phenomena can
be explained using the SV equation) (I0/I) = KSV[A] + 1 Where [A]
molar concentration of the analytes KSV is the SV constant (Mꢃ1
lL 58.43% for 100 lL 4.86% for
l
l
emission bands more structurally organized.
solvatochromic.
2 is therefore
)
I and I0 are the intensity of the complex after addition of analyte
and initial intensity of the complex. The calculated quenching con-
stant (KSV) value for TNP of 2 is 1.542 ꢂ 104 Mꢃ1. This value is supe-
rior to the many reported Zn complexes (Table S7A). The SV plots
of the TNP and other analytes are shown in Fig. 9(a) and 9(b).
The referenced figure shows with an increase in the concentration
of TNP the SV plots deviate from linearity to bending upwards
whereas other analytes showed linear relation. This nonlinear nat-
ure pointed out the energy transfer among complexes and TNP or
dynamic quenching process. Furthermore LOD was calculated
5.3. Detection of NACs and sensing
The complex formed with d10 metal ions and the extended
p
conjugation organic pro-ligands is considered fluorescent materi-
als. Our design perspective of pro-ligand unveils pockets binds
over one zinc metal ion afforded a stable conjugated network
which prevents the fluorescence process’s deactivation. The com-
plex emission spectra were recorded at room temperature
(Fig. 6) in N,N0-dimethylformamide (DMF) solvent. 1 showed an
Fig. 4. Coordination environment of Zn metal ions in 1.
7

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
the emission spectra of 2 were recorded individually after the addi-
tion of different analytes (300
lL). After that 300 lL on TNP was
added and showed a considerable amount of quenching of fluores-
cence intensity (Fig. 10).
Now the TNP ꢅ 4-NBA > NP > m-DNB > NBZ > 2,4-DNT is the
quenching efficiencies order of various NACs against 2. This
quenching efficiency order can be explained by the efficient trans-
fer of electrons from the 2 to the nitroaromatics via p∙∙∙p interac-
tions [46]. The more the NAC’s electron-deficient the more
effective it will be on the quenching. Now Scheme S3 showed dif-
ferent NACs electron-deficient characters based on electronic
effects. From the results of quenching efficiency 4-NBA is mostly
electron-deficient because of the existence of two strong deactivat-
ing groups i.e. ANO2 and ACarboxylic (COOH) groups (strong –R
effect) whereas 4-TNP is solvated in solution; thus the negative
charge density over the phenoxy oxygen atom cannot get delocal-
ized through the benzene ring [69]. As predicted nitrobenzene
(NBZ) is found to possess lesser quenching efficiency. 2,4-DNT is
the least quenching efficiency among all the NACs. A schematic
representation of the electron transfer mechanism from 2 to all
NACs shown in Fig. S23B. NACs with electron-deficient property
can gain an electron from an excited ligand state [70,71] verified
by molecular orbital theory [72,73]. Fig. S23B shows that the 1,4-
derivative (4-NBA) is more effective than the 1,3-derivative (m-
DNB) explained by the common resonance effect. The carbon atom
containing the nitro group in 4-NBA gains a positive charge facili-
tating an electron’s movement from the 2. However with m-DNB
the positive charge occurs in the carbon atom next to the ANO2
group carrying carbon. This condition may inhibit transferring an
electron from the 2 to these systems responsible for the least
Fig. 5. (a) The ORTEP diagram of 2 (30% ellipsoid probability of the independent
crystallographic unit).
Table 3
Possible non-covalent interaction parameters.
Components
Bond distance (Å)
Bond angle (°)
C21-H21CꢁꢁꢁꢁꢁN6
3.482(9)
3.713(1)
3.483(8)
3.249(5)
3.313(1)
125.377(7)
97.031(5)
136.495(5)
157.024(4)
125.364(5)
C10-H10Cꢁꢁꢁꢁꢁ
C8-H8BꢁꢁꢁꢁN6
p
C11-H1BꢁꢁꢁꢁꢁN7
C10-H10Aꢁꢁꢁꢁꢁ
p
upon the incremental addition of 10
to the 2 and simultaneously measured the fluorescence intensity
(Fig. S23A). It was found that for TNP is 0.0985 ppm
lM TNP (30–300 lL) solution
2
(Table S7B). This LOD value is superior and comparable to various
reported literature based on TNP sensing (Table S7A). The interfer-
ence study further verified the detection of TNP. In an experiment
Table 4
Decay profile of steady-state and time-resolved photoluminescence.
Entry of compounds
kex (nm)
kem (nm) (DMF)
kem (nm)
ɸ
(ɸ)
v2
(Solid-state)
–
–
DMF
0.0067
0.0482
Solid-state
0.0029
0.0047
(H2L1)
(H2L2)
330
330
421
413
–
–
Fluorescence (DMF)
kex [nm]
kem [nm]
(ɸ)
s1 [ns]
s2 [ns]
s3 [ns]
<
s> [ns]
-
1
370
370
456
0.0347
4.37
(20.5%)
2.85
15.4
(1.2%)
0.55
0.709
(78.1%)
8.2
4.41
1.31
1.16
2
482
0.0398
1.50
(4.2%)
(95.1%)
(0.6%)
Fluorescence
(Solid-state)
1
370
370
419
438
–
–
9.25
(31.8%)
11.8
83.8
(11.9%)
30.5
0.77
(56.2%)
0.70
64.62
30.48
1.32
1.34
2
(9.6%)
(74.6.9%)
(15.7%)
Fig. 6. Ligand-centerred fluorescence emission spectra of (a) pro-ligands and (b) complexes.
8

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
Fig.7a-7b. Fluorescence emission spectra of complexes in presence of NB and other solvents
Fig. 8. (a) Emission spectra of 2 upon incremental addition of 1 mM solution of trinitro phenol (TNP) up to 300
addition of 300 L 1 mM solution of each analytes individually.
lL and (b) the extent of quenching in bar diagram of 2 upon
l
Fig. 9. (a) The SV plot of 1 mM solution of TNP and (b) other nitro analytes.
quenching occurrence [69]. The TNP efficient quenching is
explained via possible stacking and H-bonding interactions
produced readily form strong OAHꢁ ꢁ ꢁO hydrogen bonds with com-
plex 2 [73]. The possible stacking and H-bonding interactions
p
—p
p—p
between analytes and the host materials which induces the elec-
tron transfer from the excited complex 2 to electron-deficient
TNP [73,73]. The six-member aromatic rings present in the ligand
sketched out for better visualization in Scheme 2.
5.4. FRET and PET mechanism
part involves
p p interactions with picrate ions [73]. Due to the
ꢁ ꢁ ꢁ
increasing number of nitro groups the acidic character of TNP
being highest over other NACs. Thereby it has the most remarkable
tendency to release proton forming picrate ion. The anions thus
We have further investigated the FRET process [74] which con-
tains such quenching fluorescence. The FRET mechanism occurs
when the resonance energy of the luminescence sensor is passed
9

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
Fig. 10. Recorded fluorescence intensity of 2 after addition of individual analytes.
After that same amount of TNP was added [Per set Black: Initial intensity of 2 Pink:
Intensity after addition of analytes and Red: After addition of TNP].
Fig. 11. Spectral overlap between emission spectrum of 2 with absorption spectra
of all NACs.
NACs [80,81]. The HOMO and LUMO energy profile of 2 and TNP
have shown that the LUMO energies are in good harmony with
TNP LUMO energies where the maximum quenching is observed
(Fig. 12). On excitation the electron from HOMO of 2 went to LUMO
while it cannot return to complex HOMO during emission. It hops
to the TNP LUMO (Fig. 12) which results in fluorescence quenching
with the highest efficiency. However the order of observed effi-
ciency of quenching is not in competition with other NACs LUMO
energies.
Again to determine the solution phase binding mechanism and
mode of interactions of TNP with 2 we have performed 1H NMR
titration with varying concentrations of TNP in DMSO d6
(Fig. S24A) at room temperature. NMR study revealed a drastic
change in the aromatic protons’ spectral pattern and downfield
shifting and broadening. 1H NMR spectra of 2 (after the addition
of TNP) the chemical shift of the proton of TNP was observed near
8.57 ppm (Fig. S24B). It is observed that aromatic protons are grad-
ually shifted to the downfield region with an increase in the con-
centration of TNP. Since TNP is strong electron-deficient it has
to the non-emissive analyte. The fluorophore and analyte are like
each other so the analyte’s electronic absorption band has suffi-
cient overlap with the fluorophore emission band [75]. The highest
quenching effect of TNP is shown by the spectral overlap of 2 emis-
sions with the maximum absorption of all the nitro analytes
Fig. 11. The degree of overlap influences the extent of energy trans-
fer and increases its sum with increasing overlap. Fig. 11 shows a
significant spectral similarity between the emission of 2 and TNP
absorption thus supporting the overall quenching efficiency for
TNP relative to other analytes. FRET is the right way of describing
2 high sensitivity and selectivity towards TNP.
The essential mechanism for fluorescence quenching of 2
against NACs is not just FRET. The PET [74] explains the quenching
of 2 fluorescence in another way. It allows the presence of low-
lying unoccupied p*-orbitals to accept an electron from the excited
state fluorophore for successful fluorescence quenching [35,76–
78]. The degree of PET for a given analyte is expressed in the effi-
ciency of fluorescence quenching emissions. Analyte electron defi-
ciency and the lowest unoccupied orbital energy level are
fundamental reasons for significant fluorescence quenching.
Therefore we have performed theoretical DFT calculations for com-
plex 2 and different NACs wherein relative HOMOs and LUMOs
energies were calculated using density functional theory at the
B3LYP/LANL2DZ level of theory (Table S8) [79]. Although 4-NBA
is the highest deficient electron analyte over TNP this explains
TNP highest efficiency fluorescence quenching over other NACs.
The different efficiencies of fluorescence quenching could be
attributed to 2 higher LUMO energies than the NACs which is kept
a driving force for electron transfer from 2 to electron-deficient
withdrawn electron-density from the zinc complex during
p-
complex formation and shifts protons toward the de-shielded
region. The other peaks remain practically constant in each step
of this NMR titration [82]. It was further confirmed that during
p-complex formation the complex is never decomposed.
6. Concluding remarks
The syntheses and characterization of two new Zn dicyanamide
complexes with 1D frameworks were described in this article. X-
ray diffraction confirmed that complexes are iso-structural. TGA
investigated the stability of metal–organic frameworks and
showed that the complexes are stable at room temperature. The
steady-state and time-resolved fluorescence spectra were per-
formed in the DMF medium where 2 is exceptionally fluorescent
over 1. The quenching efficacy of the nitro analytes is TNPꢅ4-NB
A > NP > m-DNB > NBZ > 2,4-DNT while the fluorescence intensity
of 1–2 is significantly quenched in the presence of NBZ. Interest-
ingly 2 can easily detect TNP in the presence of different epNACs
due to possible p—p stacking and H-bonding interactions. The SV
equation applies to calculate the quenching efficiency constant
(KSV) of nitroaromatics (TNP). A detection limit of 2 was also calcu-
lated and compared with literature reported Zn complexes. The
efficient quenching by TNP can be attributed to the (a) electrostatic
interactions through charge-transfer complex (b) PET by DFT and
Scheme 2. Possible
proton of TNP and complex 2.
p—p stacking and H-bonding interactions between the acidic
10

D. Majumdar, S. Dey, A. Kumari et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 254 (2021) 119612
Fig. 12. (a) Calculated HOMO and LUMO energies of different NACs by DFT (b) Fluorescence quenching mechanism via possible pathway of electron transfer of 2.
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sitivity 2 applied as a sensor for detecting TNP in DMF by turn-off
fluorescence response. Therefore the synthesized complex played a
significant role in removing the possibility of terrorist attacks for
nitroaromatics (TNP) that are explosive and harmful to living
organisms and the environment.
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& editing, Investigation, Software, Visualization, Formal analysis.
Annu Kumari: Software, Visualization. Tapan Kumar Pal: Soft-
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Dipankar Mishra: Supervision, Writing - review & editing, Investi-
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Declaration of Competing Interest
[5] (a) M.-L. Zhang, D.-S. Li, J.-J. Wang, F. Fu, M. Du, K. Zou, X.-M. Gao, Structural
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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a flexible
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Acknowledgements
This research work did not receive any specific grant from fund-
ing agencies in public commercial or non-profit sectors. Dr. Dipan-
kar Mishra gratefully acknowledges the financial grant sanctioned
by UGC New Delhi in his favour vide Minor Research Project (F.
PSW-232/15-16(ERO). Dr. Tapan Kumar Pal acknowledges to
PDPU/ORSP/R&D/SRP/2019-20/1361/1.
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